A method for ascertaining the presence or absence of leakage in the discharge prevention system of an evaporated fuel treatment device is described in Japanese Patent Application Kokai No. Hei 7-12016. In this method, when the detected internal pressure of evaporated vapor in the tank is a negative pressure of a predetermined value or greater relative to atmospheric pressure, this indicates that purging is being performed in a normal evaporated fuel treatment system during ordinary engine operation; accordingly, it is determined that there is no leakage of evaporated fuel from the evaporated fuel treatment system, and that the system is therefore operating normally. In cases where, for example, the internal pressure of the fuel tank remains stationary for a predetermined period of time in the vicinity of atmospheric pressure when such a "normal" determination is obtained in the above-mentioned process, a negative pressure diagnostic process is performed assuming that there is a possibility of leakage; in this case, the discharge prevention system is placed under a negative pressure, and the presence or absence of leakage in the tank system is ultimately determined from the negative pressure maintenance capability.
Furthermore, Japanese Patent Application Kokai No. Hei 9-317572 describes an evaporated fuel treatment device which is equipped with a bypass valve that bypasses the pressure adjustment valve in the charging passage connecting the fuel tank and the canister, and which separately ascertains the presence or absence of leakage in the tank system on the fuel tank side of the bypass valve and in the canister system on the canister side of the bypass valve. A determination of the presence or absence of leakage in the tank system is accomplished as follows: immediately after the engine is started, the bypass valve is opened so that the tank pressure is caused to move toward atmospheric pressure. If the shift in the tank pressure at this time is greater than a predetermined value, it is determined that the tank system is normal with no leakage. If there is leakage in the tank system, then the pressure in the fuel tank prior to starting is more or less equal to atmospheric pressure; accordingly, the shift in pressure is small.
As consideration for the environment has become more important, there has been a demand for stricter criteria in determining the presence or absence of leakage. However, the internal pressure in a fuel tank constantly changes due to various factors such as the temperature of the fuel, the degree to which surplus fuel is returned from the engine space, the load conditions of the vehicle, and vibration, etc. As a result, difficulties have been encountered in the accurate detection of the presence or absence of leakage caused by minute holes.
If there is frequent lighting of a warning lamp, etc., due to the erroneous detection of leakage in cases where there is actually no leakage, this results in a lowering of the practical utility of the vehicle. On the other hand, if no leakage is detected in cases where leakage is actually occurring, evaporated fuel continues to be released into the atmosphere.
Accordingly, there is a need for an evaporated fuel treatment device that can correctly detect the presence or absence of leakage caused by minute holes (e.g., holes with a diameter in the range of 0.5 mm).